Control system for frequency converters



Dec. 8, 1936. R. NOTVES1; 2,063,824

CONTROL SYSTEM FOR FREQUENCY CONVERTERS Filed April 27, 1934 Patented bee. s, 1936 g Q 2,063,824

UNITED STATES PATENT OFFICE 2,063,821

CONTROL SYSTEM FOR FREQUENCY CONVERTERS 1mm Notvest. Indianapolis, Ind.,, aasignor to J. D. Adams Manufacturing Company, Indianapolis, Ind., a corporation of Indiana Application April 21, 1934, serial, No. 722,638 1 Claim. (Cl. 219-4) My invention relates to arc-welding, and parbroader aspects, however, my invention is not ticularly to an arc-welding process employing an limited to any particular form of apparatus. alternating-current arc. In the accompanying drawing; .Fig. 1 is a dia- Alternating currents of the frequencies (25- grammatic illustration of the preferred form of 5 or BO-cycle) commonly used in distribution lines apparatus .used in practicing my invention; and 5 in this country are relatively unsatisfactory for Fig. 2 shows a curve illustrating the relation bearc-welding. This is believed to be due to the tween arc-stability and frequency. fact that the current passes through its zero value The induction-type frequency converter which I sojslowly as to permit de-ionization of the gases I prefer to employ is, structurally, a polyphase inin the vicinity of the arc, thus rendering the arc duction motor having a wound rotor the phases 10 highly unstable. Attempts have been made to of which are brought out through collector rings remedy this condition by using a high-frequency and brushes. Conveniently, the stator is em- (IOOO-cycle or over) current either as the weldployed as the primary and the rotor as the secing current itself or as superposed upon a lowondary of the frequency converter.

l5 frequency (SO-cycle or under) welding current Asis well known, the polyphase current supplied merely to maintain ionization. Neither of these to the stator windings of such a machine creates processes has proven very satisfactory commera rotating magnetic field, and the relative rocially, the formerprincipally because it required tational speed of this field and the rotor deterheavy and expensive apparatus to generate the mines the frequency of the secondary current in high-frequency, high-amperage welding current, the rotor windings. For example, if the rotor 0 and the latter for several reasons. is held stationary, the frequency of the secondary It is the object of my invention to provide an current will equal that of the primary current; arc-welding process which can be carried out with while if the rotor is driven at a rotational speed apparatus lighter and less expensive than that equal to that'of the primary magnetic field but now necessary in commercial processes of directin the opposite direction, the frequency of the 25 current arc-welding and also lighter and less exsecondary current will be double that of the pripensive than that heretofore required for the mary current. production of high-frequency welding current. The ordinary 60-cycle current of distribution A further object of my invention is to produce, lines can be readily converted to a frequency by apparatus of relatively light weight and ecosatisfactory for my use by employing such an in- 30 nomical construction, a welding are having a staduction-type frequency converter-and by rotating bility equal to or greater than that of a direct.- the rotor in a reverse direction at twice the speed current welding arc of corresponding voltage. of the rotating magnetic field produced by the I have discovered that the stability of an alterstator windings, thus generating a secondary curhating-current welding arc is not a straight-line rent of 1B0-cycle frequency. Such an arrange- 35 function of the frequency. If stability, representment lends itself to the use of standard-parts ed by ordinates, is plotted against frequency, repand thereby lowers the cost of the apparatus.

resented byabscissae, the resultant curve has a In the drawing. a three-Phase f q y 1 relatively gradual slope at frequencies below 120 verter is, denoted at Ill and is shown as directcycles per second and above 200 cycles per secconnected to a drivingmotor H by a shaft I2. 40 ond, but has a very steep slope over the range The motor ii is preferably an induction motor between 120 and 200'cycles per second. Staconnected to the same three-D pp y lines bility co-ordinate with that of a direct-current s th s which s pply, u nt t t stat r f welding arc of corresponding voltage and the frequency-converter l0; and, in order that perage is obtained at a frequency of about 150 the speed of the converter rotor may be a little 45 cycles per second. Therefore, if stability equal less than twice that of the magnet field D to that of a direct-current arc is the only deced by the stator windings of t e v t r. sideratum, 150-.cycle current can be used. For e converter "I h tw e s many po a the reasons hereinafter set forth, however, I prefer motor H. In the drawing, therefore, I have in- 5 to use a welding current having a frequency of dicated the frequency converter ID as having about 180 cycles per second. I eight poles and the motor H as having four My preferred method of obtaining alternating poles, as an example of this relation. To obtain current of the desired frequency from the generin the converter a magnetic field which rotates ally available -cycle supply is through the use in a direction opposite to that of rotor-rotation,

so, of an induction-type frequency converter, In its the connections of the supply lines II to either to the motor II or the converter-stator are reversed. In the drawing, such reversal is shown in the connections between the supply lines It and the motor i I.

The frequency converter ll may be an ordinary form of three-phase induction motor wound to produce welding current of the desired voltage and amperage. As indicated in the drawing, the rotor windings are star-connected, and since usually only a single-phase current is desired for welding two of the rotor terminals are connected respectively to the welding electrode I1 and the work ll, while in the example shown the third rotor terminal is not used. Thus, the welding current is the vector sum of the currents in two series-connected phases of the rotor. This arrangement is merely illustrative, however, as it is possible, if desired. to employ only a single rotor phase or to use all three rotor phases and convert the three-phase current thus produced to single-phase current by additional apparatus; or

the rotor might be specially wound to produce single-phase current. Obviously, the manner in which single phase welding current is derived from a polyphase rotor will govern the characteristics of the rotor winding. The illustrated arrangement utilizing series-connected phases of a polyphase rotor is preferred, at least for low-quantity production of the welding apparatus, because of the economy resulting from the use of standard parts.

For the purpose of regulating the voltage of the welding current I may employ two multitapped auto-transformers 2| and 2i connected respectively across two phases of the three-phase supply circuit. The line wire connected in common tothe two auto-transformers is also connected to one terminal of the stator windings of the frequency-converter II, and switch mechanism is provided for connecting the other two stator terminals to any desired pair of corresponding taps of the two auto-transformers 2| and 2!, either to step up or to step down the voltage.

The drawing shows the switch mechanism Just referred to as two arcuate series of stationary contacts 22 and 23 and a swinging switch arm 24 carrying two Jointly movable contacts II and 2| co-operating respectively with the two series of fixed contacts 22 and 22 and connected to two of the stator terminals on the converter. The fixed contacts 22 are connected in order to the taps of the auto-transformer 2| and the contacts 23 are similarly connected to the taps of the auto-transformer 2| in such a way that any pair of fixed contacts 22 and 22 simultaneously.

the eifect of the current supplied to them, it is desirable, when starting the apparatus, to throw the motor upon the line first in order that it may begin to rotate before current is supplied to the stator of the converter. To this end I may employ a line switch of the type indicated d agrammatically in the drawing. This switch has three jointly movable contact arms 8|, connected respectively to the three wires of the supply line, and two sets of stationary contacts II and 32. The set of fixed contacts |I is connected to the motor II, and is arranged to be engaged by the arms 8| in their closing movement before such arms engage the other set of fixed contacts 82.

such latter contacts being connected through the voltage-regulating mechanism to the stator of the converter ll.

In starting the apparatus, the contact arms H are moved into engagement with the fixed contacts II, but are not moved far enough to engage the other set of fixed contacts 32. The resultant supply of current to the motor Ii causes it to drive the rotor of the converter II; and after the motor has come up to speed, closing. movement of the line switch may be completed to throw the converter stator upon the line.

I have referred above to the relation between the stability of an alternating-current welding arc and the frequency of the welding current. This relation is indicated by the curve shown in Fig. 2, in which stability is measured as the maximum arc-length which can be maintained between a steel plate and a commercial form of coated welding electrode. The curve of Fig. 2 represents stabilities obtained with a welding current of amperes and 40 volts.

It is to be noted that the maximum arc-length which can be obtained with 60-cycle current of the stated voltage and amperage is in the neighborhood of V; of an inch. Increasing the frequency from 60 to 120 cycles increases the maximum arc-length by about Y of an inch, or to about 96 of an inch. A further increase of frequency to 180 cycles effects a marked increase in stability, for it increases the maximum arc-length from about of an inch at 120 cycles to about 1% inches at 180 cycles, Above about 180 cycles, the per-cycle increase in stability which accompanics increased frequency gradually decreases, and above about 200 cycles again becomes relativ Lv slight.

A direct current welding arc of 120 amperes and 40 volts has a maximum maintainable length ofabout to%ofaninch. 'lhisisfive orsix times as great as the. maximum arc-length obtained with alternating current of the standard I 60-cycle frequency, and approximatdy double the maximum arc-length obtained with alternating current of i20-c'ycle frequency; but only approximately half the maximum length obtainable with ISO-cycle frequency. Stability co-ordinate with that of a direct-current arc is obtained at frequencies in the neighborhood of to cycles p r second.

- While frequencies in the neighborhood of 150 cycles per second would produce, in the practice of my process, a welding are having a stability substantially equal to that of a direct-current arc of the same voltage and amperage, I prefer to use a welding current of slightly less than 180 cycles, not only because of the increased arcstability obtained but also because it permits the use of standard electrical equipment. A commercial four-pole GO-cycie motor operates at between 1750 and 1800 R. P. 11., and this speed will produce, in an eight-pole frequency converter, a secondary current of between and cycles per second-a frequency highly satisfactory for my purpose.

While frequencies above 180 cycles per second would produce an arc of further increased stability, not only does the increase in stability gradually lessen as the frequency thence increases. but such further increased stability is not needed in practice. Moreover, the higher the frequency, the more elaborate and expensive becomes the equipment necessary to produce it. I do not regard the increased arc-stability obtainable at frequencies above 250 cycles of sumcient importance at present to justify the increased cost of the equipment necessary to produce currents of such frequencies. Frequencies between 150 and 250 cycles are therefore preferred; and of such frequencies, I believe 180-cycle to be most satisfactory.

In an induction-type frequency converter of the kind described, a part of the energy represented by the secondary current is derived from the driving means and a part by transformer action within the converter. Specifically, the ratio of the energy resulting from transformer action to the energy derived from the driving means equals the ratio of the original frequency to the increase in frequency. In changing -cycle current to 180-cyc1e current in accordance with my process the increase in frequency is cycles and, therefore, two-thirds of the electrical energy in the welding circuit is derived from the driving motor and one-third results from transformer action within the converter. a

In an ordinary motor-generator 'set, whether used to produce direct current or alternating current,-,the electrical energy of the welding circuit is derived entirely from the motor. In other words, the current drawn from the supply line is all used in the motor to create mechanical energy, and the mechanical energy thus derived is then converted into electrical energy by the generator. A commercial form of welding machine (motor-generator set) for producing 200 ampere, 40 volt direct current from alternating current supply lines employs a- 15 H. P. motor. In the practice of my process, I can obtain 200-volt, 40 ampere welding current with the. use of a 10 H. P. motor, since only two-thirds of the energy in the welding circuit is derived from the'motor. ,This is of great importance, as it not only reduces the cost of the welding machine but also reduces its weight; and as welding machines are nearly always portable in character, any reduction in weight is a distinct advantage.

I claim as my invention:

In a frequency converter system for use in a welding circuit having a driving motor and an induction generator whose armature is actuated by the driving motor, both motor and generator being supplied from a common power circuit, the combination of a power circuit, a motor circuit, a generator field circuit connected to the stator of the generator, switch means to insure connecting the power circuit first to the motor circuit and then to the generator circuit, and means of adjusting the generator field strength and the electrical output from the rotor of the generator, and an armature circuit associated with the rotor of the generator.

ROBERT NO'IVEST. 

